In a press call on Thursday, US Housing and Urban Development Secretary Julián Castro, US Rep. Luis Gutiérrez (D-Illinois), and activist Dolores Huerta hammered Sanders repeatedly for his votes against a 2007 immigration-reform bill and his support for a 2006 bill allowing people who live in the US without permission to be detained indefinitely.

Castro, widely rumored to be on the short list of Clinton's potential vice-presidential picks, criticized Sanders for touting a pro-immigrant record that "doesn't exist."

"It's disappointing to see Sen. Sanders talk up a record that just doesn't exist," Castro said. "And that's really what I want to emphasize of what we said: Sen. Sanders consistently wasn't there when we needed him the most."

Gutiérrez acknowledged Sanders' support for the US Senate's 2013 immigration-reform bill, one of the most highly contested congressional battles of the Obama administration. But Gutiérrez said that that wasn't enough.

"I never remember Bernie saying, 'By the way, can you use a helping hand if there's something we can do together. Is there something we can work on together on immigrant and Latino issues?'" Gutiérrez said. "Never once."

He added:

Being with us today doesn't make up for years of absence. Now that Sanders is running for president in 2016, I'm happy. But many of us have long memories that help us recognize who we can count on and who let us down when we needed them.

Thursday's press call came as Clinton and Sanders attempt to woo Latino voters before Saturday's Nevada caucuses, where the voting bloc is expected to play a key role.

Clinton's campaign has been on the ground for months attempting to shore up support and train local volunteers to canvas and conduct caucuses in English and Spanish.

Bernie Sanders. Evan Vucci/AP Photo

But Sanders isn't conceding Nevada or the immigration debate.

Over the past several weeks, the senator's campaign slammed Clinton for asserting in 2014 that the wave of immigrant children fleeing strife in Latin America should be sent back to "send a message."

"Secretary Clinton is wrong. We should never put children back into harm's way to 'send a message' to anyone," Sanders' Hispanic media director, Arturo Carmona, said in a statement. "That's why Sen. Sanders has put forward a serious immigration plan that focuses on protecting kids and keeping families together. It is time for Secretary Clinton to do the same."

Carmona, during a call with reporters in December, said that the campaign was already seeing a major uptick in interest from Hispanic volunteers.

"Latinos are really gravitating towards our campaign, and the numbers are changing every day, especially in states like Nevada," Carmona said.

"We had thousands of Latino and Spanish-speaking volunteers doing a call a couple weeks back with one of our celebrity endorsers, George Lopez," he continued. "We got nearly 1,000 volunteers in one hour."